1. Field of the Invention
The present invention relates to an optical device for focusing a laser beam, for example a substantially elliptic and astigmatic laser beam.
The invention also relates to a lens and to an optical element for focusing a laser beam emitted by a source of emission, in particular a laser diode. The invention also relates to an apparatus and a method for assembling the above device.
Preferably, but not exclusively, the device—or the lens, or the optical element—of the invention are adapted to be used on a small low-priced optical reader such as, for example, a portable reader of optical codes.
In this description and following claims, the term “optical reader” refers to any device adapted to acquire information relating to an object (for example, distance, volume, dimensions, or its identification data) through the acquisition and the processing of a luminous signal diffused by it, while the term “optical code” refers to a code (such as, for example, a bar code or a bidimensional code, or the like) adapted to univocally identify the objects on which it is provided.
By way of example and for the purpose of making the following description clearer, optical-code readers will be explicitly referred to.
2. Discussion of the Prior Art
An optical-code reader comprises, in its simplest embodiment, a source of emission of a luminous signal for illuminating an object, photodetecting means for collecting the luminous signal diffused by the illuminated object and generating an electric signal proportional to it, and computing means for processing and treating the generated electric signal so as to acquire the desired information. Usually, downstream of the source of emission is a focusing lens, adapted to focus the luminous signal emitted by the source of emission at a fixed distance.
As already known, particularly in small low-priced optical readers, there is the need of using inexpensive and small-sized focusing devices. For this purpose, the use of semiconductor laser diodes as source of emission is still widespread.
Nevertheless, although the use of a laser diode allows having advantages in terms of costs and overall dimensions of the reader, it presents the disadvantage of not allowing a reliable reading of optical codes having distances and inclinations very different from one another (this situation frequently occurs, for example, in handling systems for the distribution and sorting of objects that are identifiable by means of optical codes).
This disadvantage is essentially related to the fact that the laser beam emitted by a laser diode is divergent, with an elliptic and astigmatic profile; this causes the presence, in the focusing point, of a circular spot and, immediately before and after, an elliptic spot with the major axis of the ellipse that, downstream of the focusing point, is rotated by 90° with respect to its position upstream of the focusing point.
It is evident that such a type of beam shape is not adapted to carry out reliable readings of codes located at distances which are different with respect to the focusing one, except for a small interval.
The presence of an emission laser with elliptic profile can be particularly disadvantageous if the codes to be read are bar codes; to get a reliable reading, in this case the dimension of the spot in the direction of reading must be smaller than the minimum width among that of the narrower bar and of the narrower space; often, this is not possible with an elliptic-profile beam.
Thus, in order to overcome said disadvantages, laser-diode optical readers are equipped with further optical elements intended for controlling the dimensions of the laser beam in the focusing point and in its surroundings. In particular, these elements consist of a circular or rectangular (or any other suitable shape) aperture stop (or diaphragm) located into the optical device downstream of the focusing lens (or between the source of emission and the focusing lens), which is intended for introducing diffraction in the emission laser beam.
In fact, it is known that the passage of a luminous wave front through an aperture causes diffraction in the wave front; this means that all the internal points of the aperture and on its profile act as sources of spherical waves all in phase with one another. The diffraction pattern (spot) that can be seen at any distance from the aperture is the coherent sum of all these spherical waves. By combining the focusing effect of the laser beam caused by the use of the focusing lens, with the diffraction effect caused by the interposition of an aperture between the lens and the focal point (or between the source of emission and the lens), a spot is obtained in the focusing point and in its surroundings, the shape of which is the diffraction pattern corresponding to the aperture, scaled in its dimensions by a factor depending on the enlargement or reduction carried out by the lens.
Essentially, the main effect of the diffraction from one aperture is both that of increasing the dimensions of the laser beam in the focusing point with respect to those that it would have as a consequence of the simple geometrical convergence of the rays carried out by the focusing lens, and that of keeping the spot collimated and substantially with the same profile for a greater time interval, and finally, that of giving a more suitable shape to the spot for a possible reading of optical codes (in fact, by properly choosing the dimensions of the aperture, the reversal between major and minor axes of the ellipse before and after the focusing point vanishes). In this way, there is the advantage of increasing the reading field (depth of field) and of eliminating an excessive focusing of the spot, which would otherwise detect the imperfections of the support on which the code is printed.
Nevertheless, the introduction of a diaphragm of the above type into the optical device inevitably implies greater dimensions of the device itself; in addition, the relative positioning between diaphragm and focusing lens requires a special attention for the purpose of obtaining the desired effects of focusing and diffraction and of allowing a reliable reading. Additionally, since the type of diaphragm to be used (in particular, the aperture shape and dimension) is different on the various applications depending on the required efficiency, on the desired focusing distance, on the symmetry or asymmetry of the generated laser spot, etc., it is necessary to have a set of different diaphragms, each one being intended for a specific application; this implies an increase in the production and labour costs.
The technical problem at the basis of the present invention is that of providing an optical device for focusing a laser beam which on the one side should be constructively simple, of small dimensions and inexpensive, so as to be assembled inside low-priced and small-sized optical readers, and on the other side which should allow obtaining all the advantages resulting from the introduction of diffraction in the emission laser beam, thus overcoming at least some of the disadvantages described above with reference to the prior art.